An internal combustion engine according to the first aspect of the present invention has been known from, for example, Japanese Patent Laid-open No. Hei 7-63065. In this internal combustion engine, the operation axes of an intake valve and an exhaust valve are so set as to skewly intersect a plane containing the axis of a cylinder bore. A spark plug is attached to a cylinder head while being located coaxially on the axis of the cylinder bore.
However, in the internal combustion engine disclosed in Japanese Patent Laid-open No. Hei 7-63065, part of a fuel-air mixture flowing into the combustion chamber via the intake valve port flows toward the side of the exhaust valve port and the spark plug. Therefore, in the case of performing an operation in which the intake gas charging effect is enhanced by overlapping a part of the intake valve opening period with a part of the exhaust valve opening period, part of the fuel-air mixture would blow by through the exhaust valve port in an unburned state, thereby increasing the fuel consumption, and it would be necessary to after-treat the unburned fuel-air mixture. In addition, where a tip end portion of the spark plug is disposed to face the intake valve port, there would be a problem (particularly, at the time of warming up) in that the spark plug becomes sooty.
An internal combustion engine according to the second aspect of the present invention has been known from, for example, Japanese Patent No. 2537205. In the internal combustion engine, the operation axes of a pair of exhaust valves are set substantially parallel to the axis of the cylinder bore. The operation axes of a pair of intake valves are set to skewly intersect the operation axes of the exhaust valves.
However, in the internal combustion engine disclosed in Japanese Patent No. 2537205, a fuel-air mixture flows skewly into the combustion chamber via the intake valve port when the intake valve is opened, so that diffusion of the fuel-air mixture in the combustion chamber cannot be said to be favorable, and the combustion efficiency also cannot be said to be sufficient. In addition, since the intake valve is inclined with its stem end spaced away from the axis of the cylinder bore, the side surface of the cylinder head on the side where the intake port is provided must be set at a position comparatively remote from the axis of the cylinder bore. Therefore, the degree of freedom in laying out the intake system connected to the cylinder head is lowered.